IR: use inlineClassRepresentation in getInlineClassUnderlyingType

Looking for the primary constructor manually doesn't work if it's
private in the other module on JVM, because private declarations are
skipped in IrLazyClass.

compiler/tests-gen/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -15867,6 +15867,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/inlineClasses/passInlineClassWithSpreadOperatorToVarargs.kt");
         }
 
+        @TestMetadata("privateConstructorFunInterfaceMultiModule.kt")
+        public void testPrivateConstructorFunInterfaceMultiModule() throws Exception {
+            runTest("compiler/testData/codegen/box/inlineClasses/privateConstructorFunInterfaceMultiModule.kt");
+        }
+
         @TestMetadata("propertyLoweringOrder.kt")
         public void testPropertyLoweringOrder() throws Exception {
             runTest("compiler/testData/codegen/box/inlineClasses/propertyLoweringOrder.kt");